When compared with four-stroke engines, ported, two-stroke, opposed-piston internal combustion engines have acknowledged advantages of specific output, power density, and power-to-weight ratio. For these and other reasons, after almost a century of limited use, increasing attention is being given to the utilization of opposed-piston engines for motive power in a wide variety of modern vehicles.
Per FIG. 1 an opposed-piston, two-stroke engine 8 includes at least one cylinder 10 with a bore 12 and longitudinally displaced intake and exhaust ports 14 and 16 machined or formed in the cylinder, near respective ends thereof. Each of the intake and exhaust ports includes one or more circumferential arrays of openings in which adjacent openings are separated by a solid portion of the cylinder wall (also called a “bridge”). In some descriptions, each opening is referred to as a “port”; however, the construction of a circumferential array of such “ports” is no different than the port constructions in FIG. 1. Fuel injection nozzles 17 are secured in threaded holes that open through the sidewall of the cylinder. Two pistons 20, 22 are disposed in the bore 12 with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is referred to as the “intake” piston because of its proximity to the intake port 14. Similarly, the piston 22 is referred to as the “exhaust” piston because of its proximity to the exhaust port 16. Preferably, but not necessarily, the intake piston 20 and all other intake pistons are coupled to a crankshaft 30 disposed along one side of the engine 8, and the exhaust piston 22 and all other exhaust pistons are coupled to a crankshaft 32 disposed along the opposite side of the engine 8. A gear train (not shown) couples the crankshafts and includes an output shaft that provides motive power to drive a vehicle. Other representative opposed-piston engine constructions are described in U.S. Pat. No. 1,683,040; U.S. Pat. No. 2,031,318; U.S. Pat. No. 8,485,161 B2; and U.S. Pat. No. 8,539,918 B2.
During operation of a two-stroke, opposed-piston engine, such as the engine 8 of FIG. 1, pairs of pistons move in opposition in the bores of ported cylinders such as the cylinder 10. In a compression stroke, as two opposed pistons move toward each other in a cylinder bore, a combustion chamber is formed in the bore, between the end surfaces of the pistons. Fuel is injected directly into the volume of the combustion chamber when the pistons are at or near respective top center (“TC”) locations in the bore. The fuel is injected through fuel injector nozzles mounted on the sidewall of the cylinder. The fuel mixes with air admitted into the bore. As the air-fuel mixture is compressed between the piston end surfaces, the compressed air reaches a temperature that causes the fuel to ignite. Combustion follows. Combustion timing is frequently referenced to “minimum volume” of the combustion chamber, which occurs when the piston end surfaces are in closest mutual proximity. In some instances injection occurs at or near minimum volume; in other instances, injection may occur before minimum volume. In any case, in response to combustion the pistons reverse direction and undergo a power stroke. During the power stroke, the pistons move away from each other toward bottom center (“BC”) locations in the bore. As the pistons reciprocate between top and bottom center locations they open and close ports formed in respective intake and exhaust locations of the cylinder in timed sequences that control the flow of air into, and exhaust from, the cylinder.
The related applications describe recent improvements to opposed-piston engines which incorporate compression-release functionality into construction and operation of the engines. In this regard, compression release functionality involves the release of compressed air from a cylinder other than through its exhaust port and in the absence of combustion. One example is compression-release braking. Compression-release braking is a particularly useful feature for vehicles such as medium-duty and heavy-duty trucks because it uses engine operations to slow vehicle speed instead of (or in addition to) friction brakes. The designs for compression-release braking for opposed-piston engines involve the exhaustion of compressed air from between the piston end surfaces while fuel injection is suppressed. Work performed in transporting and compressing the air is not returned to the crankshafts, thereby slowing the engine, which slows the vehicle. The compressed air is released by way of a valve acting through the side of the cylinder at a location between the intake and exhaust ports of the cylinder. As taught in U.S. Pat. No. 8,746,190 B1, the released compressed air can be stored in an accumulator and released therefrom to supplement work performed by various engine components during normal engine operation.
As the designs for opposed-piston internal combustion engines advance and lead to improved performance with engine configurations, the returns of investment will begin to diminish. It is therefore useful and desirable to consider hybridization of opposed-piston engine systems by incorporation of stored energy that can be activated during engine operation to supplement the work enabled by internal combustion alone, which will introduce a new factor to increase the engine's efficiency. The rewards of such hybridization would be increased to the extent that the stored energy could be replenished by the engine during operation.
One hybrid engine system that has been proposed for vehicle use may be described as an air/gasoline hybrid in which compressed air is generated and stored during unassisted gasoline operation and then released to assist gasoline operation of the engine or to power the engine solely with air. (Hybrid Air An innovative petrol full-hybrid solution PSA PEUGEOT CITROEN Press Release Jan. 22, 2013). It would be beneficial in terms of improved performance to consider the hybridization of opposed-piston engine systems by combining pneumatic and combustion capabilities to power the engines.
The compression-release braking constructions and the air storage and release capability described in related U.S. Pat. No. 8,746,190 B1 are combined to enable valve-controlled operation for transporting stored compressed air from an accumulator into a channel through which air is provided to the intake ports of the opposed-piston engine. The provision of compressed air may, for example, supplement work performed by a supercharger during normal combustion operation, thereby improving fuel consumption. This engine system performs as a mild hybrid with two modes of operation: combined compressed-air/combustion and combustion alone. However, without the capability of operating the engine in a compressed-air-only mode, the full hybrid potential is unrealized.